Add skip_discriminant to skip discriminant comparison in derived PartialEq implementations
#116
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rustc wants to use derivative to simplify
PartialEqderives, but encounters a pretty significant perf hit because thederivative(PartialEq)impl will add a discriminant comparison first, unlike our hand-rolled implementations.This PR adds a
PartialEq = "skip_discriminant"mode to skip generating an initial discriminant comparison so that rustc can derive more tailoredPartialEqimplementations for derivative. This is proven to work.